Methods for synthesis of liquid crystals

ABSTRACT

Novel platform molecules and polymerizable mesogens made therefrom; novel synthetic pathways for making such platform molecules and polymerizable mesogens.

PRIORITY DATA

[0001] The present application claims the benefit of the followingprovisional applications, all filed Jan. 23, 2001: Ser. No. 60/263,387;Ser. No. 60/263,392; Ser. No. 60/263,388.

GOVERNMENT RIGHTS CLAUSE

[0002] The U.S. government has certain rights in this invention pursuantto grant number NIDCR 1 P01 DE11688.

FIELD OF THE INVENTION

[0003] The application relates to new, less costly methods for makingnovel platform molecules and polymerizable mesogens.

BACKGROUND OF THE INVENTION

[0004] Photocurable resins which are transparent or translucent,radioopaque, have good workability, and have good mechanical strengthand stability are useful in medical, dental, adhesive, andstereolithographic applications.

[0005] Low polymerization shrinkage is an important property for suchresins. In dental applications, the phrase “zero polymerizationshrinkage” typically means that the stresses accumulated during curingdo not debond the dentin-restorative interface or fracture the tooth orrestorative, which can result in marginal leakage and microbial attackof the tooth. Low polymerization shrinkage also is important to achieveaccurate reproduction of photolithographic imprints and in producingoptical elements.

[0006] Another advantageous property for such resins is maintenance of aliquid crystalline state during processing. For comfort in dentalapplications, the resin should be curable at “room temperature,” definedherein as typical ambient temperature up to body temperature. Preferredcuring temperatures are from about 20° C. to about 37° C. Mesogens whichhave been found to polymerize in a relatively stable manner at suchtemperatures are bis 1,4[4′-(6′-methacryloxyhexyloxy) benzoyloxy]t-butylphenylene mesogens and their structural derivatives. Thesemesogens have the following general structure:

[0007] Unfortunately known synthetic methods for producing thesemesogens are costly and have relatively low yields. As a result, themesogens have enjoyed limited commercial use. Less costly syntheticmethods are needed to produce the mesogens.

SUMMARY OF THE INVENTION

[0008] A method for producing platform molecules comprising:

[0009] providing a first phenylene ring comprising a first functionalgroup at a para-position to a second functional group;

[0010] providing a second phenylene ring comprising a third functionalgroup at a para-position to a fourth functional group;

[0011] providing a third phenylene ring comprising a desired substituentand comprising and a first functionality at a para-position to a secondfunctionality; and

[0012] reacting said first functional group with said firstfunctionality, producing at least a first ester bond between said firstphenylene ring and said third phenylene ring; and

[0013] reacting said third functional group with said thirdfunctionality, producing at least a second ester bond between saidsecond phenylene ring and said third phenylene ring, thereby producingplatform molecules comprising a first terminal functionality at positionpara- to said first intervening ester bond and a second terminalfunctionality at a position para- to said second intervening ester bond,wherein at least one functionality selected from the group consisting ofsaid first terminal functionality and said second terminal functionalityis other than a polymerizable group;

[0014] wherein, when both said first terminal functionality and saidsecond functionality are polymerizable groups, said desired substituentprovides sufficient steric hindrance to achieve a nematic state at roomtemperature while suppressing crystallinity at room temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The application provides novel platform molecules, novelpolymerizable mesogens, novel methods for using the platform molecules,and novel intermediates and synthetic pathways for making the platformmolecules and polymerizable mesogens.

[0016] The Mesogens

[0017] The mesogens of the present application have the followinggeneral structure:

[0018] wherein X and Y are selected from the group consisting ofterminal functionalities and polymerizable groups. In platformmolecules, X and Y are terminal functionalities. In polymerizablemesogens, X and Y are polymerizable groups. Terminal functionalities andpolymerizable groups are further defined below; and,

[0019] R² is a desired substituent, preferably a “bulky organic group,”defined herein as an organic group having a bulk greater than R₁ and R₃,whereby, when both X and Y are polymerizable groups, said bulk isadapted to provide sufficient steric hindrance to achieve a nematicstate at room temperature while suppressing crystallinity at roomtemperature. The result is effective rheology and workability at roomtemperature. Suitable R² groups generate asymmetry in the packing of themolecules, and include, but are not necessarily limited to alkyl groupshaving from about 1 to 6 carbon atoms and aryl groups. Preferred R²groups include, but are not necessarily limited to alkyl groups havingfrom about 1 to about 4 carbon atoms and phenyl groups. More preferredR² groups are methyl groups, t-butyl groups, isopropyl groups, secondarybutyl groups, and phenyl groups. Most preferred R² groups are methylgroups and t-butyl groups; and

[0020] R¹and R³ are selected from groups less bulky than R², preferablyselected from the group consisting of hydrogen atoms and methyl groups,depending upon the relative bulk of R¹, R³ and R².

[0021] As used herein, the phrase “terminal functionalities” refers to Xand Y where the referenced molecules are platform molecules. “Terminalfunctionalities” are defined as protective groups and precursors topolymerizable groups, which generally comprise functionalities thatreadily react with “polymerizable groups” to form reactive ends.Suitable terminal functionalities independently are selected from thegroup consisting of hydroxyl groups, amino groups, sulfhydryl groups,halogen atoms, and “spacer groups”, defined herein as selected from thegroup consisting of H—(CH₂)_(n)—O— groups, Cl(CH₂)_(n)—O— groups,Br(CH₂)_(n)—O— groups, I(CH₂)_(n)—O—, wherein n is from about 2 to about12, preferably from about 2 to about 9, more preferably from about 2 toabout 6, and most preferably 6, and the CH₂ groups independently can besubstituted by oxygen, sulfur, or an ester group; provided that at least2 carbon atoms separate said oxygen or said ester group. Most preferredterminal functionalities are hydroxyl groups.

[0022] Where the mesogen is a polymerizable mesogen, X and/or Y are“polymerizable groups,” defined as groups that may be polymerized bynucleophilic addition, free radical polymerization, or a combinationthereof. Preferred polymerizable groups are polymerizable by Michaeladdition. Michael addition requires the addition of a nucleophile and anelectron deficient alkene. Groups suitable for polymerization by Michaeladdition include but are not necessarily limited to the examples foundin A. Michael, J Prakt. Chem. [2] 35, 349 (1887); R. Connor and W. R.McClelland, J. Org Chem., 3, 570 (1938); and C. R. Hauser, M. T.Tetenbaum, J. Org. Chem., 23, 1146 (1959), all of which are incorporatedby reference herein.

[0023] Examples of suitable polymerizable groups include, but are notnecessarily limited to substituted and unsubstituted alkenyl estergroups comprising a polymerizable unsaturated carbon-carbon bond,wherein said alkenyl group has from about 2 to about 12 carbon atoms,preferably from about 2 to about 9 carbon atoms, more preferably fromabout 2 to about 6 carbon atoms. In one embodiment, said substitutedalkenyl ester groups comprise at least one halogen atom selected fromthe group consisting of chorine atoms, bromine atoms, and iodine atoms.Preferred alkenyl esters are acryloyloxy groups, methacryloyloxy groups,acryloyloxy alkoxy groups and methacryloyloxy alkoxy groups. Morepreferred polymerizable groups include, but are not necessarily limitedto cinnamoyloxy groups, acryloyloxy groups, methacryloyloxy groupscomprising an alkyl moiety having from about 2 to about 12 carbon atoms,thiolalkyloxy groups comprising an alkyl moiety having from about 2 toabout 12 carbon atoms, preferably from about 2 to about 9, morepreferably from about 2 to about 6, and most preferably 6 carbon atoms.Because assymetry suppresses crystallinity while maintaining a nematicstate, it is preferred for X and Y to be different groups.

[0024] Most preferred polymerizable mesogens are bis1,4[4′-(6′-(R,R⁴)-oxy-A-oxy)benzoyloxy]R²-phenylene mesogens. Thesemesogens have the following general structure:

[0025] This structure is similar to the structure of the platformmolecules except that X and Y are replaced by polymerizable groupswherein:

[0026] A is selected from the group consisting of alkyl groups andmethyl-substituted alkyl groups having from about 2 to about 12 carbonatoms, preferably having from about 2 to about 9 carbon atoms, morepreferably having from about 2 to about 6 carbon atoms, and mostpreferably having about 6 carbon atoms; and

[0027] R and R⁴ are polymerizable groups, including but not necessarilylimited to nucleophiles and groups comprising at least one electrondeficient alkene. Suitable nucleophiles include, but are not necessarilylimited to ester groups, organic acid groups, amine groups, hydroxylgroups, and sulfhydryl groups. More preferred polymerizable groupscomprise electron deficient alkenes. Suitable electron deficient alkenesindependently are selected from the group consisting of substituted andunsubstituted alkenyl ester groups comprising a polymerizableunsaturated carbon-carbon bond, wherein said alkenyl group has fromabout 2 to about 12 carbon atoms, preferably about 6 carbon atoms. Inone embodiment, said substituted alkenyl ester groups comprise a halogenatom selected from the group consisting of chlorine atoms, bromineatoms, and iodine atoms. Preferred alkenyl esters are acryloyl groupsand methacryloyl groups. Again, because assymetry suppressescrystallinity while maintaining a nematic state, it is preferred for Xand Y to be different groups. One end of a polymerizable mesogen alsomay comprise a bridging agent, in which case R² may also be hydrogen orgroup less bulky than a methyl group, due to the inherent assymmetry ofthe dimer molecule. Dimers are discussed more fully below.

[0028] In a preferred embodiment, R² is either a t-butyl group or amethyl group, A is a hexyl group, and one of R and R⁴ is selected fromthe group consisting of an acryloyl group and a methacryloyl group.

[0029] In a preferred embodiment, a proportion of X and/or Y (or Rand/or R⁴) comprises a crystallization retardant. A “crystallizationretardant” is defined as a substituent that retards crystallization ofthe monomers without suppressing the T_(n->isotropic) (the nematic toisotropic transition temperature). The proportion of X and/or Y (or Rand/or R⁴) that comprises a crystallization retardant preferably issufficient to suppress crystallinity of the mesogenic material,particularly at room temperature for dental applications, and tomaintain flowability of the mesogenic material under the particularprocessing conditions. Suitable crystallization retardants include, butare not necessarily limited to halogen atoms. Exemplary halogen atomsare chlorine, bromine, and iodine, preferably chlorine. Typically, theproportion of the crystallization retardant required is about 3-50 mole%, more preferably 10-15 mole %, and most preferably about 14 mole % orless.

[0030] Depending on the sample preparation, the volumetricphotopolymerization shrinkage of these materials at room temperaturevaries from about 0.9 to about 1.7%, which is a factor of 6-4×improvement over commercially available blends containing2,2-bis[p-(2′-hydroxy-3′-methacryloxypropoxy)phenylene] propane(“bis-GMA”). Preferably, the volumetric polymerization shrinkage isabout 3 vol. % change or less, more preferably about 2 vol. % change orless.

[0031] Mesomers of higher temperature nematic stability are “mesogenicdimers,” formed by reacting X and Y with opposite ends of a bridgingagent. Examples of suitable bridging agents include, but are notnecessarily limited to dicarboxylic acids (preferably α,ω-carboxylicacids) having from about 4 to about 12 carbon atoms, preferably fromabout 6 to about 10 carbon atoms, and oligodialkylsiloxanes preferablycomprising alkyl groups having from about 1 to about 3 carbon atoms,most preferably methyl groups.

[0032] New Synthetic Pathways to Make the Mesogens

[0033] In the past, polymerizable mesogens having the foregoingstructure were synthesized by a multistep process (“Scheme 1”), as shownbelow:

[0034] In Scheme 1, molecular ends containing the outer aromatic groupsand the alkyl groups were produced first and then coupled to the centralaromatic group by diaryl ester bonds. Specifically, the alkali phenoxidesalt of p-hydroxybenzoic acid-ethyl ester nucleophile attacked the6-hydroxy 1-chloro hexane with the aid of iodide catalyst to produce the6-hydroxyhexyloxybenzoic acid (after hydrolysis of the ethyl ester) by aprocedure that yielded at best 70% product. Although ratherstraightforward, the commercial potential of this synthesis has beenlimited by the use of the 6-hydroxy 1-chlorohexane. The reaction is runin acetone over several days and requires significant workup. Thereaction also produces only about a 40% overall yield, at best, andrequires column separation to separate monosubstituted fromdisubstituted material.

[0035] The present application provides new synthetic pathways that userelatively low cost materials to synthesize a central aromatic componentcomprising end groups that are easily reacted with the desiredpolymerizable groups. The methods are qualitative, produce high yields,the products are easily purified (preferably by crystallization), andmany of the products are more stable than bisalkenes, which must bestabilized against polymerization.

BRIEF SUMMARY OF THE PROCESSES

[0036] According to the present application, functionalities on aphenylene ring at para-positions (preferably hydroxyl groups) form esterlinkages with one of two functionalities in para-positions on two otherphenylene rings. The result is three-ring platform molecules havingterminal functionalities. One or both of the terminal functionalitiesmay be coupled with polymerizable groups, preferably a nuceleophileand/or an electron deficient alkene-containing group, to producepolymerizable mesogens.

[0037] Preparation of Molecular Ends and Coupling to Central AromaticGroup

[0038] In a first embodiment (Scheme 2), the molecular ends of themesogen (outer aromatic and alkyl groups) are prepared and coupled tothe central aromatic group by diaryl ester bonds. This synthetic pathwayis illustrated and described in detail below:

[0039] Exemplary “platform molecules” are illustrated in (6), above.

[0040] To summarize Scheme 2, bis1,4[4″-(6′-chloroalkyloxy)benzoyloxy]R²-phenylene, preferably bis1,4[4″-(6′-chlorohexyloxy) benzoyloxy]t-butylphenylene, is converted tothe analogous bis ω-hydroxy or ω-hydroxy chloro compound. Thehydroxy-compound (the platform molecule) may be terminated with one ormore polymerizable groups. Preferred polymerizable groups arenucleophilic and electron deficient groups, most preferablyindependently selected from the group consisting of acryloyl groups,methacryloyl groups, and cinnamoyl groups.

[0041] More particularly:

[0042] (1) 4-nitrobenzoic acid is dissolved in an excess of the desired1,6-dihydroalkane, preferably 1.6-dihydroxyhexane, in the presence of asuitable esterification catalyst. Suitable catalysts include, but arenot necessarily limited to titanium alkoxides, tin alkoxides, sulfonicacid, and the like. A preferred catalyst is Ti(OBu)₄. The dissolutionoccurs at atmospheric pressure at a temperature of from about 120° C. toabout 140° C., with stirring. If excess alcohol is used, the majorityproduct is the 6-hydroxyalkyl ester of 4-nitrobenzoic acid plus some bis1,6-(4-nitrobenzoyloxy)alkane, preferably 1,6-(4-nitrobenzoyloxy)hexane. The byproduct water is removed using suitable means, preferablyunder vacuum during the course of the reaction.

[0043] (2) One or more suitable solvents are added to the reactionmixture, along with alkali salts of diols. Suitable solvents include,but are not necessarily limited to aprotic solvents in whichnucleophilic attack is preferred. Examples include, but are notnecessarily limited to dimethyl sulfoxide (DMSO), dimethyl formamide(DMF), dimethyl acetamide (DMAC), hexamethyl phosphonamide (HMPA). Apreferred solvent is dimethylsulfoxide (DMSO), which is environmentallysafe and relatively inexpensive. Suitable salts comprise cationseffective to displace hydrogen and to produce the mono-cation salt ofthe alkanediol, preferably the nucleophilic monosodium salt ofhexanediol, in the presence of excess alkyldiol, preferably hexanediol.Preferred salts include, but are not necessarily limited to NaH orKOBu^(t). The salt of the alkane diol, preferably hexane diol, thendisplaces the activated nitro group to produce4-(1-hydroxyalkyloxy)benzoic acid (1-hydroxyalkyl ester) and some of thedimeric compound. A preferred product is 4-(1-hydroxyhexyloxy)benzoicacid (1-hydroxyhexyl ester) and some of the dimeric compound. See N.Kornblum et al., J. Org. Chem., 41(9), 1560 (1976), incorporated hereinby reference (nucleophilic displacement of nitro-group).

[0044] (3) The mixture from (2) is diluted with an aqueous base andheated to completely cleave the aryl-alkyl ester to produce the desired4-(6′-hydroxyakyloxy)benzoic acid by precipitation subsequent toacidification. Suitable aqueous bases include, but are not necessarilylimited to inorganic bases, a preferred base being aqueous sodiumhydroxide. Suitable acids include, but are not necessarily limited toinorganic acids, a preferred acid being hydrochloric acid. In apreferred embodiment, 4-(1-hydroxyhexyloxy)benzoic acid (1-hydroxyhexylester) is diluted with aqueous sodium hydroxide and then acidified usinghydrochloric acid to produce 4-(6′-hydroxyhexyloxy)benzoic acid. Thesupernatant contains sodium chloride and nitrite, which can be removedand recovered by vacuum evaporation of the solvent. In a preferredembodiment, the solvents evaporated are DMSO, hexanediol and water,which may be discarded. DMSO and hexanediol can be recovered from thewater phase by known distillation procedures.

[0045] (4) In a preferred embodiment, for small scale procedures, aquantitative conversion of the 4-(6′-hydroxyalkyloxybenzoic acid to4-(6′-chloroalkyloxy)benzoyl chloride is accomplished by mixing withthionyl chloride diluted in a suitable solvent, preferably toluene, inthe presence of pyridine base. In a preferred embodiment,4-(6′-hydroxyhexyloxy)benzoic acid is converted to4-(6′-chlorohexyloxy)benzoyl chloride in this manner. On a larger scale,the foregoing reaction is implemented with simple addition of SOCl₂ andventing of the byproduct SO₂ and HCl.

[0046] (5) The highly reactive 4-(6′-chloroakyl)benzoyl chloride iscoupled to a hydroquinone bearing the desired bulky group, R². In apreferred embodiment, 4-(6′-chlorohexyl)benzoyl chloride is mixed atroom temperature with t-butyl hydroquinone in ether with pyridine, usedas catalyst and as a base to take up released HCl, to form bis1,4[4″-(6′-hydroxyhexyloxy)benzoyloxy]t-butylphenylene. The reaction isquantitative and produces a high yield of the desired product. Inaddition, the bis 1,4[4″-(6′-chloroalkloxy)benzoyloxy]R²-phenylene,preferably bis 1,4[4″-(6′-chlorohexyloxy)benzoyloxy]t-butyl phenylene,is easily purified from the reaction mixture by crystallization. Inaddition, the bischlorocompound is stable and need not be stabilizedagainst polymerization (as must bis-alkene compounds).

[0047] (6) The bischlorocompound is hydrolyzed to the platform molecule,preferably bis 1,4[4″-(6′-chlorohexyloxy)benzoyloxy] t-butylphenylene,by simple heating in an aprotic solvent in the presence of water andpotassium bromide [R. O. Hutchins and I. M. Taffer, J. Org. Chem, 48,1360 (1983)]. Again, the reaction is quantitative with the product beingpurified by recrystallization. The reaction can be stopped atintermediate times to produce any desired mixture of monofunctional anddifunctional alcohol molecules. In addition, the chloro-terminatedmolecules can be converted to the more reactive iodo-terminated speciesby simple exchange with NaI in acetone.

[0048] (7) The dialcohol or mixed alcohol/alkyl chloride is easilyreacted with one or more polymerizable groups, preferably Michaeladdition reactants. In a preferred embodiment, one or more of thedialcohol ends is reacted with alkenyl chlorides to form reactivealkenyl esters, which can have any ratio of alkenyl ester, halide, oralcohol termini. The ratio can be adjusted to adjust the crosslinkdensity and the liquid crystal transition temperatures.

[0049] Selective Ether Cleavage

[0050] In a preferred embodiment, 4-alkoxy benzoyl chloride, preferablycommercially available 4-methoxy benzoyl chloride, is reacted with ahydroquinone substituted with a desired R² group to produce thecorresponding aromatic ester, bis 1,4[4-alkoxybenzolyoxy]phenylene,preferably bis 1,4[4-methoxybenzolyoxy]phenylene. The reaction takesplace in the presence of an appropriate HCl scavenger and solvent.Suitable HCl scavengers include, but are not necessarily limited toaromatic and aliphatic amines, with a preferred HCl scavenger beingpyridine. The pyridine also may be used in combination with a trialkylamines having from about 2-4 carbon atoms, preferably triethyl amine.

[0051] In a second “step,” the alkoxy group is cleaved to result in areactive hydroxyl group while leaving the aromatic ester and thus thetriaromatic mesogen structure intact. See M. Node et al., J. Org. Chem.,45, 4275 (1980)] (FIG. 7a). incorporated herein by reference. Nodesuggests that the methyl ether of bis 1,4[4-methoxybenzolyoxy] phenylenecan be selectively cleaved in the presence of a nucleophile, preferablya thiol, and a Lewis acid, such as aluminum chloride, to produce bis1,4[4-hydroxybenzoyloxy]phenylene. [See M. Node et al., J. Org. Chem.,45, 4275 (1980)] (“Node”), incorporated herein by reference. However,Node describes cleaving methyl ethers in the presence of aliphaticesters—not in the presence of aromatic esters. In initial experimentsusing the conditions described in Node, the more unstable aromaticesters underwent significant ester cleavage because the product complexremained in solution where additional reaction can occur.

[0052] Surprisingly, selective cleavage of the aliphatic ether in thepresence of the aromatic esters was induced at low temperatures usingmuch higher methyl ether concentrations than those described in Node.Using high concentrations of the ether and much lower concentrations ofthe nucleophile induced a “complex”—containing the dihydroxy productwith intact aromatic ester bonds—to precipitate from the reactionmixture at short reaction times as the complex was formed. Theprecipitated complex decomposed to the desired dihydroxy compound byreacting the complex with water and/or alcohol.

[0053] Suitable ethers for use in the reaction include, but are notnecessarily limited to alkyl ethers, having from about 1 to about 8,preferably 1 to 4 carbon atoms. A most preferred ether is methyl ether.Suitable nucleophiles for use in the reaction include, but are notnecessarily limited to aliphatic thiols. Preferred nucleophiles areliquid alkanethiols, which typically have 11 carbon atoms or less. Amost preferred nucleophile is ethane thiol.

[0054] Preferably, a minimum amount of thiol is used to dissolvealuminum chloride in the presence of the ether and a solvent. A mostpreferred embodiment uses at least 1 mole of thiol per mole of alkylether, preferably 2 moles of thiol per mole of alkyl ether. A mostpreferred embodiment uses 7 mmol of the methyl ether per ml of ethanethiol.

[0055] The aluminum chloride to ether ratio should be 4:1 or more, asthis appears to be the ratio needed for complexation. At ratios ofaluminum chloride to thiol of above 5, more of the complex will stay inthe solution before saturation occurs thus resulting in aromatic estercleavage and reduced yield. The use of less aluminum chloride willresult in an incomplete cleavage of the methyl ether. The use of morealuminum chloride, in excess of 4 to 1, has shown no effect inincreasing the reaction rate, but slight excesses such as, 4.5 to 1 cancompensate for residual water in the system.

[0056] Suitable solvents for use in the reaction are halogenatedsolvents, preferably chlorinated solvents, most preferablydichloromethane. The solvent concentration can range from a molar excessof from about 3 to about 7, preferably about 5 or more, in relation tothe nucleophile (thiol), as needed to keep the solution in a slurry asprecipitate forms. However, dichloromethane above a 5 molar excessshould be added slowly as the reaction proceeds since high initialconcentration of the methylene chloride will hinder the reaction rate.

[0057] The reaction preferably is started under dry conditions at about0° C. but can be allowed to warm to room temperature (˜25° C.) as itproceeds. The reaction should not go above room temperature or estercleavage can occur.

[0058] Upon increasing methyl ether concentration to 35× theconcentrations used by Node, the solubility limit of the product complexwas exceeded, permitting the complex to crystallize out of the reactionmixture before the aromatic esters had an opportunity to cleave.Quantitative yields were obtained when the complex crystallized directlyfrom the reaction mixture, effectively removing the molecule fromfurther reaction that would form side products:

[0059] The diphenolic platform mesogens can be lengthened by reactingadditional 4-methoxy benzoyl chloride with bis1,4[4′-methoxybenzoyloxy]t-butylphenylene to produce the dimethoxycompound with four or five aromatic rings, depending upon the reactantratios. Cleavage with Lewis acid and thiol produces the respectiveelongated diphenolic platform molecules:

[0060] The phenolic end group(s) are esterified by acyl chlorides, thusproviding a route to polymerizable mesogens. For example, reaction ofC0[H,TB,H](OH)₂ with methacryloyl chloride formed the monoester whichwas coupled to bifunctional sebacoyl chloride to form an alkyl diesterlinked, methacrylate terminated liquid crystalline monomer,{C0[H,TB,H](MeAcry)(O)}₂ (seb) with T_(n->I) of 145° C. and a T_(g) of25° C. This monomer had no tendency to crystallize since the synthesisyielded three different isomers with differing mutual orientation oft-butyl groups. The material is highly viscous, however, makingprocessing close to room temperature, and thus T_(g), somewhatinconvenient.

[0061] Formation of Dimers

[0062] Preferred non-reactive dimeric and polymeric derivatives ofC₆[H,TB,H] type mesogenic cores are much more unlikely to crystallize[S. Lee et al., Macromol., 27(14), 3955 (1994)]. In addition, blends ofnon-reactive dimeric with monomeric derivatives (C₆[H,TB,H](Me)₂generated a phase diagram with isotropic, isotropic+nematic and finally,at the lowest temperatures, a nematic phase. Adding polymer to themonomer substantially increases T_(n->n+I).

[0063] Briefly, in order to make the dimer molecule, a second mesogenic,platform molecule, 1,4[4′-hydroxybenzoyloxy]t-butylphenylene,C0[H,TB,H](OH)₂, is synthesized by coupling p-anisoyl chloride witht-butyl hydroquinone and then cleaving the methoxy end groups, asdescribed above, preferably using ethanethiol and aluminum chloride.This molecule can be further extended by reaction with p-anisoylchloride and the same methoxy cleavage reaction. Fully aromatic diphenolterminated mesogens of any length can be thus produced.

[0064] Reaction of C0[H,TB,H](OH)₂ with a less than stoichiometricamount of methacryloyl chloride forms the monoester and diester. Themonoester and diester are washed away from the diphenol startingmaterial with methylene chloride and the monoester is separated from thediester as an insoluble solid by diluting the methylene chloridesolution into hexane.

[0065] The monoester can be coupled to bifunctional sebacoyl chloride toform an alkyl diester linked, methacrylate terminated liquid crystallinemonomer, {C0[H,TB,H](MeAcry)(O)}2 (seb) with T_(->I) of 145° C. and aT_(g) of 25° C. This monomer has no tendency to crystallize since thesynthesis yields three different isomers with differing mutualorientation of t-butyl groups. However, processing close to roomtemperature, and thus T_(g), is inconvenient because of the highviscosity of the material.

[0066] The following is a ChemSketch 4 rendition of the minimum energyconformation of {C0[H,TB,H](MeAcry)(O)}₂ (seb). As expected the moststable conformation is an extended form with a very high molecularlength to width ratio which is likely to form high T_(n->I) liquidcrystal monomers.

[0067] A minimum energy conformation of a preferred mesogenic dimer isdecanedioic acidbis-(4-{2-tert-butyl-4-[4-(2-methyl-acryloyloxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester{C0[H,TB,H](MeAcry)(O)}₂(seb):

[0068] Alternately, the partially or completely methacryloylated oracryloylated versions of decanedioic acidbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester and decanedioic acidbis-(4-{2-tert-butyl-4-[4-(2-methyl-acryloyloxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester are made as illustrated below:

[0069] The first reaction product in the above figure is a novelalkylenedioicbis-(4-{2-alkyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester having the following general structure:

[0070] wherein

[0071] R⁴ has from about 2 to about 20 carbon atoms, preferably fromabout 2 to about 12 carbon atoms, and most preferably from about 6 toabout 12 carbon atoms.

[0072] the alkyl substituent on the central aromatic group of thearomatic ends includes, but is not necessarily limited to t-butylgroups, isopropyl groups, and secondary butyl groups. Most preferred aret-butyl groups; and,

[0073] V and W are selected from the group consisting of terminalfunctionalities and polymerizable groups. In platform molecules, V and Ware terminal functionalities. In polymerizable mesogens, V and/or W arepolymerizable groups.

[0074] The same procedures may be used to make mesogens having thefollowing general structure:

[0075] wherein

[0076] R⁵ and R⁶ are selected from the group consisting of hydrogen,halogen, alkyl groups having from about 1 to 6 carbon atoms, and arylgroups; and,

[0077] V and W independently are selected from the groups comprisingpolymerizable groups and terminal functionalities.

[0078] Suitable terminal functionalities independently are selected fromthe group consisting of hydroxyl groups, amino groups, and sulfhydrylgroups. Most preferred terminal functionalities are hydroxyl groups.

[0079] Suitable polymerizable groups may be polymerized by eithernucleophilic addition, free radical polymerization, or a combinationthereof. Preferred polymerizable groups are polymerizable by Michaeladdition. Michael addition requires the addition of a nucleophile and anelectron deficient alkene. Groups suitable for polymerization by Michaeladdition include but are not necessarily limited to the examples foundin A. Michael, J. Prakt. Chem. [2] 35, 349 (1887); R. Connor and W. R.McClelland, J. Org. Chem., 3, 570 (1938); and C. R. Hauser, M. T.Tetenbaum, J. Org. Chem., 23, 1146 (1959), all of which are incorporatedby reference herein.

[0080] Examples of suitable polymerizable groups include, but are notnecessarily limited to substituted and unsubstituted alkenyl estergroups comprising a polymerizable unsaturated carbon-carbon bond,wherein said alkenyl group has from about 2 to about 12 carbon atoms,preferably from about 2 to about 9 carbon atoms, more preferably fromabout 2 to about 6 carbon atoms. Preferred alkenyl esters areacryloyloxy groups and methacryloyloxy groups. V and W may be the sameor different, depending upon the application. In a preferredapplication—a dental application—V and W comprise terminal alkenylgroups.

[0081] These alkylenedioicbis-(4-{2-alkyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)esters are novel compounds, and may be used as “platform molecules,” orpolymerizable mesogens. A most preferred alkylenedioicbis-(4-{2-alkyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester is decanedioic acidbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester.

[0082] In order to make the dihydroxyaromatic terminated mesogens, 1,4bis(4′-hydroxybenzoyloxy) t-butylphenylene orbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl) ester is dissolved in a solvent at a ratio of about 10ml. solvent per gram. The material is dissolved in the solvent under aninert gas, preferably dry nitrogen. Suitable solvents are heterocyclicbases, with a preferred solvent being pyridine. This first mixture isdiluted with a chlorinated organic solvent, preferably methylenechloride, in an amount equal to the volume of pyridine.

[0083] A second mixture is formed by dissolving an alkyloyl chloride ina chlorinated organic solvent at a ratio of about 10 ml solvent per gramof alkyloyl chloride. A preferred chlorinated organic solvent ismethylene chloride. The alkyloyl chloride comprises an alkyl portionhaving from about 2 to about 20 carbon atoms, preferably from about 6 toabout 20 carbon atoms, more preferably from about 6 to about 12 carbonatoms, and most preferably is sebacoyl chloride. This second mixtureincludes at least some of benzoquinone inhibitor, suitableconcentrations being from about 1 to about 100 ppm, with a preferredconcentration being about 10 ppm. The second mixture is added slowly tothe first mixture with stirring, preferably with a syringe through asuba seal. After about 24 hours at room temperature, a precipitate isseen. The solvent, preferably methylene chloride and pyridine, arepumped off.

[0084] Any remaining pyridine is converted to a salt using a suitableacid, preferably hydrochloric acid, and the salt is removed by washingwith water. Water is filtered off from the remaining white precipitate.Residual water is removed using a suitable solvent, preferably acetone,to dissolve the remaining precipitate, which is then stirred with asuitable amount of magnesium sulfate. The solution is dried down and adissolved in a chlorinated organic solvent, preferably methylenechloride (DCM), is added to dissolve the solid. After 24 hours at roomtemperature the unreacted 1,4 bis(4′-hydroxybenoyloxy) t-butylphenylenecrystallizes out of solution as a white precipitate and separated fromthe mixture. The solution was then placed in the freezer overnight anddecanedioic acidbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester precipitates out of solution. Silica and basic alumina may beadded to absorb any remaining methacrylic acid or carboxylic acidterminated products.

[0085] Aromatic terminated mesogens (herein called “mesogenic dimers”),such as the foregoing, are used as a diluent and blended with thealiphatic terminated mesogens (herein called polymerizable mesogen) toform the polymerizable mixture. The quantity of mesogenic dimer in theblend will vary depending upon the dimer and its impact on transitiontemperature, final product, etc.

[0086] Reaction of Dimethyl Amine or Dichloro TerminatedOligodimethylsiloxanes with the Mono Methacrylate Ester of 1,4[4′-hydroxybenzoyloxy]t-butylphenylene

[0087] Molecules with high temperature stability can be prepared byreacting dimethyl amine or dichloro terminated oligodimethylsiloxaneswith the mono methacrylate ester of1,4[4′-hydroxybenzoyloxy]t-butylphenylene, as shown below:

[0088] In this embodiment, the mesogenic platform molecule1,4[4′-hydroxybenzoyloxy]t-butylphenylene is further extended byreaction with p-anisoyl chloride and subsequent ether methyl groupcleavage with aluminum chloride and ethane thiol. Fully aromaticdiphenol terminated mesogens of any length can be thus produced.Reaction with acryloyl or methacryloyl chloride forms the monoester,which can be coupled to reactive aliphatic or siloxane oligomers to formpolymerizable liquid crystals with reactive ends.

[0089] Formation of Alkoxy Terminal Functionalities

[0090] In order to produce alkoxy functionalities, an excess of anisoylchloride is mixed with a desired 1,4 bis(4′-hydroxybenzoyl oxy)-R²phenylene, (preferably a t-butylphenylene) in an excess of pyridine andtriethyl amine (about a 10:1 ratio) with stirring under nitrogen forseveral hours, preferably about 4 hr. The pyridine is removed undervacuum, and the mixture is extracted into ethyl ether. Aminehydrochloride is removed by vacuum filtration and the remaining solidsare washed with a suitable solvent, such as water and acetone. Theproduct had a melting point of 222-224° C. and the structure of themolecule was confirmed by NMR to be the aromatic dimethoxy compound.

[0091] Low Polymerization Shrinkage

[0092] The mesogens exhibit low polymerization shrinkage. Polymerizationshrinkage is measured by codissolving the monomers in dichloromethanewith 0.3 wt. % camphorquinone photoinitiator, 100 ppm benzoquinone and 1wt. % N,N′ dimethylaminoethyl methacrylate activator and subsequentlypumping off the solvent, all under yellow light. The monomers are thenpolymerized in film or droplet form in less than 1 minute by exposure toa dental curing light (Dentsply Spectrum Curing Lamp) with a significantoutput at 420 nm.

[0093] FTIR spectroscopy (Nicolet Magna-IR 560) is used to measure thedegree of cure by observing the decrease in the 1637 cm⁻¹ alkene bandvs. the aromatic internal thickness band at 1603 cm⁻¹. Thin filmmeasurements that avoid oxygen inhibition are performed by sandwichingthe monomer between polyvinylidene chloride films, which have an opticalwindow in the wavelength region of interest. The IR spectrum of soliddroplets is evaluated using a single bounce reflectance measurement. Theflat bottom surface of the droplet is pressed against the germaniumlense of a Spectra Tech Thunderdome attachment.

[0094] Polymerization of the monomers can be observed betweentransparent polyvinylidene chloride films under cross-polarized opticalmicroscopy in the heated stage of a Nikon Optimat microscope. Littlechange in the local birefringence and thus local orientation is notedupon polymerization at room temperature or upon heating to 180° C.

[0095] Fracture Toughness

[0096] Compact tension samples (ASTM E399) with known edge crack lengthare fabricated by photocuring monomer with initiator and activator insilicone molds. After polishing the surface with 600 grit polishingagent and soaking in physiologic saline at 37° C. for 24 hours thesamples are tested at room temperature under displacement control at 1mm/min until failure.

[0097] The fracture toughness of the crosslinked, amorphous glass is ashigh as possible, suitably 0.4 Mpa-m^(1/2) or higher, preferably 0.5MPa-m^(1/2) or higher, which is the same as that found for photocured,isotropic dimethacrylate based resins such as GTE resin (3M company).

[0098] Fillers

[0099] Considerable amounts of soluble impurity can be added to thepolymerizable mesogens, or a mixture comprising the polymerizablemesogens, without changing the T_(nematic->isotropic) transitiontemperature of the polymerizable mesogens. Thus, a high volume fractionof filler can be added to the polymerizable mesogens and still form acomposite that maintains desirable, low viscosity flow and lowpolymerization shrinkage characteristics at temperatures of curing.Commercial products add up to about 70-80 wt % filler. A preferredembodiment uses about 30 wt. % filler.

[0100] A variety of fillers may be used. A preferred filler isamphoteric nano-sized metal oxide particles having a diameter innanometers which is sufficiently small to provide transparency effectivefor photopolymerization but sufficiently large to provide effectivefracture toughness after photopolymerization. Substantially any “metal”capable of forming an amphoteric metal oxide may be used to form themetal oxide particles. Suitable metallic elements include, but are notnecessarily limited to niobium, indium, titanium, zinc, zirconium, tin,cerium, hafnium, tantalum, tungsten, and bismuth. Also suitable in placeof the metal in the oxide is the semi-metallic compound, silicon. Asused herein, unless otherwise indicated, the term “metal oxide” isdefined to include silicon, and the word “metal,” when used to refer tothe metal oxide is intended to also refer to silicon.

[0101] The metal oxides may be made of a single metal, or may be acombination of metals, alone or combined with other impurities or“alloying” elements, including, but not necessarily limited to aluminum,phosphorus, gallium, germanium, barium, strontium, yttrium, antimony,and cesium.

[0102] A monomeric liquid crystal (LC) containing a high volume fractionof filler nanoparticles is a highly constrained system. As a result, atleast for some monomeric species, both smectic and crystallinetransitions should be suppressed. The consequent widening of thestability range of nematic mesophase should permit the composite topolymerize at much lower temperatures than in unfilled systems,resulting in lower polymerization shrinkage.

[0103] The metal oxide nanoparticles may be prepared using any knownmethods, such as “sol-gel” techniques, direct hydrolysis of metalalkoxides by water addition, forced hydrolysis of relatively low-costmetal salts, or non-hydrolytic reactions of metal alkoxides with metalhalide salts. Examples of such procedures are shown in the followingreferences, each of which is incorporated herein by reference: W. Stöberand A. Fink, J. of Colloid and Interface Science, v. 26, 62-69 (1968);M. Z.-C. Hu, M. T. Harris, and C. H. Byers, J. of Colloid and InterfaceScience, v. 198, 87-99 (1988); M. Ocaña and E. Matijević, J. ofMaterials Research, v. 5(5), 1083-1091 (1990); L. Lerot, F. LeGrand, P.de Bruycker, J. of Materials Science, v. 26, 2353-2358 ( 1991); H.Kumazawa, Y. Hori, and E. Sada, The Chemical Eng'g. Journal, v. 51,129-133 (1993); S. K. Saha and P. Pramanik, J. of Non-CrystallineSolids, v. 159, 31-37 (1993); M. Andrianainarivelo, R. Corriu, D.Leclercq, P. H. Mutin, and A. Vioux, J. of Materials Chemistry, v.6(10), 1665-1671 (1996); F. Garbassi, L. Balducci, R. Ungarelli, J. ofNon-Crystalline Solids, v. 223, 190-199 (1998); J. Spatz, S. Mössmer, M.Mo[umlaut]ller, M. Kocher, D. Neher, and G. Wegner, Advanced Materials,v. 10(6), 473-475 (1998); R. F. de Farias, and C. Airoldi, J. of Colloidand Interface Science, v. 220, 255-259 (1999); T. J. Trentler, T. E.Denler, J. F. Bertone, A. Agrawal, and V. L. Colvin, J. of the Am.Chemical Soc., v. 121, 1613-1614 (1999); Z. Zhan and H. C. Zheng, J. ofNon-Crystalline Solids, v. 243, 26-38 (1999); M. Lade, H. Mays, J.Schmidt, R. Willumeit, and R. Schömacker, Colloids and Surfaces A:Physiochemical and Eng'g Aspects, v. 163, 3-15 (2000); and the proceduredescribed in “Sol-gel processing with inorganic metal salt precursors,”authored by “Michael” Zhong Cheng Hu, licensable via Oak Ridge NationalLaboratory under ORNL control number ERID 0456.

[0104] The application will be better understood with reference to thefollowing examples, which are illustrative only:

EXAMPLE 1 Synthesis of 4-nitrophenylenecarbonyloxy 6′-hexane-1′-ol

[0105] 60 g 4-nitrobenzoic acid (0.4 mole) was dissolved in 250 ml (2.07mole) dry hexanediol that had been fused in the reaction vessel at 165°C. 1 ml. tetrabutyltitanate catalyst was added, and the mixture wasstirred for 3 hours at 135° C. before cooling to 95° C. where stirringwas continued under dynamic vacuum for two days to remove the water ofcondensation.

[0106] The solution was extracted with 1 liter diethyl ether,centrifuged or filtered to remove the catalyst, and then washed twotimes with 500 ml 5% NaHCO₃ to remove unreacted acid and excess diol.After the ether was vacuum evaporated, the residue was dissolved in 150ml boiling ethanol to which 75 ml water was added. Upon cooling to roomtemperature bis 1,6-(4 nitrophenylene carbonyloxy)hexane precipitated as7.61 grams of a yellow powder (T_(m)=112° C.).

[0107] The remaining solution was evaporated and redissolved in 150 mldiethyl ether to which was added 75 ml hexane. After crystallization at−20° C. 4-nitrophenylene 4-carbonyloxy 6″-hexane-1′-ol (86.7 grams) wasisolated (T_(m)=32-35° C.). NMR indicated that both of these productswere greater than 98% purity.

EXAMPLE 2 Synthesis of 4-(6-hydroxyhexyloxy)phenylenecarbonyloxy6′-hexane 1′-ol

[0108] 20 ml (0.166 mole) of dry, molten hexanediol was transferred to aflask with an attached short path distillation unit. 200 ml drydimethylsulfoxide (DMSO) and then 40 ml of 1M KOBu^(t) was then added tothe diol and stirred 45 minutes at room temperature. The Bu^(t)OH and asmall amount of DMSO were distilled off under vacuum between 25-50° C.over one hour. 8 ml (0.04 mole) of dry 4-nitrophenylenecarbonyloxy6′-hexane-1′-ol was added producing a bright blue color that convertedto a yellow coloration after 2 hours.

[0109] After stirring overnight, the DMSO and excess hexanediol wasremoved by vacuum distillation at 90° C., whereupon the residue wastaken up in 200 ml diethyl ether which was washed twice with 200 ml 5%NaHCO₃ and dried with MgSO₄. After the ether was distilled away, thesolid was dissolved in a minimum amount of boiling ethanol andcrystallized at −20° C. A 75-90% yield of the desired white product wasobtained (T_(m)=30-33° C.).

EXAMPLE 3 Synthesis of 4-[6-hydroxyhexyloxy]benzoic Acid

[0110] 1.2 g (0.0037 mole) 4-(6-hydroxyhexyloxy)phenylenecarboxyoxy6′-hexane 1′-ol was heated for 8 hours at 90° C. in a solution of 0.29 g(0.0074 mole) NaOH in 4 ml water. 20 ml of water was added to the clearsolution and 0.3 ml of concentrated HCl added to precipitate the acid atpH=3-5. The white solid was filtered off and dried under vacuum toproduce a quantitative yield of the substituted benzoic acid (T_(m)=117°C.).

EXAMPLE 4 Synthesis of 4 (6′-chlorohexyloxy)benzoyl Chloride

[0111] A three times molar excess of thionyl chloride (55 ml) in toluene(300 ml) was dropwise added over 20 minutes to4-(6′-hydroxyhexyloxy)benzoic acid (60 g, 0.252 mole) suspended intoluene (600 ml) with a stoichiometric amount of pyridine (42 ml) at 0°C. The suspension was continuously stirred for another 8 hours at roomtemperature, whereupon the toluene and excess thionyl chloride weredistilled off at 70-100° C. with a slight nitrogen flow. The remainingslush of the pyridine hydrochloride and product was extracted with 11boiling hexane and mixed with 5 g basic alumina and 5 g neutral silicaand filtered hot. A 90% yield of a very light yellow4-(6′-chlorohexyloxy)benzoyl chloride liquid was obtained afterevaporation of the hexane (T_(m)<20° C.).

EXAMPLE 5 Synthesis of bis1,4[4″-(6′-chlorohexyloxy)benzoyloxy]t-butylphenylene

[0112] 65 g of 4-(6′-chlorohexyoxy)benzoyl chloride (0.23 mole) wasadded to 16.75 g (0.1 mole) of t-butyl hydroquinone dissolved in 800 mldry diethyl ether. 10 ml pyridine and 32 ml triethylamine were thenadded to this mixture. After stirring for 20 hours, the ether wasfiltered and washed two times with 200 ml 0. 1N HCl and 200 ml saturatedNaCl solution. The ether solution was then mixed with 10 g basic aluminato remove unreacted acid and 10 g neutral silica to flocculate thesuspension and dried over magnesium sulfate. The product starts tocrystallize from the ether when the solution is reduced by half. Aftercontinued crystallization at −20° C. overnight 63 g of product meltingat 95-100° C. could be obtained. Another crop of crystals was obtainedby further reducing the solution and crystallizing at −20° C. over oneweek. NMR purity was >99%.

EXAMPLE 6 Synthesis of bis1,4[4″-(6′-iodohexyloxy)benzoyloxy]t-butylphenylene

[0113] 1.15 g (0.0016 mole) bis1,4[4″-(6′-chlorohexyloxy)benzoyloxy]t-butylphenylene dissolved in 20 mlacetone was boiled under nitrogen with 8.0 g NaI in 20 ml acetone for 20hours. A quantitative yield of bis1,4[4″-(6′-iodohexyloxy)benzoyloxy]t-butylphenylene was obtained. Thematerial melted at 76° C. and was >99% pure by NMR.

EXAMPLE 7 Synthesis of bis 1,4[4″-(6′-hydroxyhexyloxy]t-butylphenylene

[0114] 36 g of bis 1,4[4″-(6′-chlorohexyloxy)benzoyloxy]t-butylphenylenewas dissolved in 750 ml of n-methypyrrolidinone (NMP) in a single neckflask. 15 g KBr and 120 ml water were then added. The flask was thenwired shut with a suba seal, and the solution was heated to 120° C. for24 hours. Upon cooling, the solution was quenched into 1500 ml water andextracted with 250 ml methylene chloride. After evaporation of themethylene chloride, the solid was extracted with 11 of ether and washedwith 11 water and dried with MgSO₄. The solution was concentrated andcrystallized at −20° C. for 3 days to yield 17 g of white productmelting at 80° C. Additional product crystallized from the solutionafter several weeks. NMR purity was >99%.

[0115] Stopping the above reaction at intermediate times yieldedmixtures of di-OH terminated, and asymmetric monochloro, monohydroxycompounds.

EXAMPLE 8 Synthesis of bis1,4[4″-(6′-methacryloyloxyhexyloxy)benzoyloxy]t-butylphenylene

[0116] 10 g (0.0165 mole) bis 1,4[4″-(6′-hydroxyhexyloxy)benzoyloxy]t-butylphenylene was dissolved in 200 ml dry methylene chloridecontaining 100 ppm benzoquinone (free radical quencher). After coolingthe above solution to 0° C. 3.2 ml (0.035 mole) distilled methacryloylchloride was then added along with 3 ml (0.037 mole) pyridine and thesolution was stirred for 24 hours in a sealed flask making no attempt toremove air from the solvent.

[0117] The solvent was vacuum evaporated and the resultant solid takenup in 250 ml ether and washed with 250 ml 0.1N H Cl and 250 ml saturatedNaCl. After drying with MgSO₄ and filtering, the solvent was evaporatedto yield 10 g of the desired product as a nematic liquid, which was >98%pure by NMR. This material could be crystallized from diethyl ether at−20° C. to form a white crystalline solid melting at 57° C.

EXAMPLE 9 Synthesis of bis1,4[4″-(6′-cinnamoyloxyhexyloxy)benzoyloxy]t-butylphenylene

[0118] 5 g (0.0825 mole) of bis 1,4[4″-(6′-hydroxyhexyloxy)benzoyloxy]t-butylphenylene was dissolved in 100 ml dry methylene chloridecontaining 100 ppm benzoquinone (free radical quencher). After coolingthe above solution to 0° C., 3.0 g (0.018 mole) cinnamoyl chloride wasthen added along with 1.4 ml (0.017 mole) pyridine, and the solution wasstirred for 24 hours in a sealed flask making no attempt to remove airfrom the solvent.

[0119] The solvent was vacuum-evaporated and the resultant solid takenup in 100 ml ether and washed with 100 ml 0.1N HCl and 250 ml saturatedNaCl. After drying with MgSO₄ and filtering, the solvent was evaporatedto yield 5 g of the desired product which was >98% pure by NMR. Thismaterial could be crystallized from diethyl ether at −20° C. to form awhite crystalline solid melting at 70° C.

EXAMPLE 10 Synthesis of bis1,4[4″-(6′-acetoxyoxyhexyloxy)benzoyloxy]t-butylphenylene

[0120] 1 g (0.0165 mole) of bis1,4[4″-(6′-hydroxyhexyloxy)benzoyloxy]t-butylphenylene was dissolved in20 ml dry methylene chloride. After cooling the above solution to 0° C.,0.27 ml (0.0037 mole) acetyl chloride was then added along with 0.3 mlpyridine, and the solution was stirred for 24 hours in a sealed flask.

[0121] The solvent was vacuum-evaporated and the resultant solid takenup in 20 ml ether and washed with 20 ml 0.1N HCl and 250 ml saturatedNaCl. After drying with MgSO₄ and filtering, the solvent was evaporatedto yield the product quantitatively at >98% purity by NMR. This materialcould be crystallized from diethyl ether at −20° C. to form a whitecrystalline solid melting at 82° C.

EXAMPLE 11 Synthesis of 1,4 Bis(4′-methoxybenzoyloxy)t-butylphenylene

[0122] Anisoyl chloride (4.93 g, 0.029 mole), t-butyl hydroquinone (2.00g, 0.012 mole) in pyridine (50 ml) and triethyl amine (3.2 ml) werestirred under nitrogen for 4 hours with the mixture eventually becomingdark orange/red. The pyridine was removed under vacuum and the mixturewas precipitated into ethyl ether (500 ml). Amine hydrochlorideprecipitated out of solution and was removed by vacuum filtration. Theether was evaporated and the slightly yellow crystals were dissolved inchloroform and extracted with slightly acidified water. The color of thecrystals was then removed by stirring over basic alumina and thecrystals were then purified by recrystallization in isopropanol. 4.8grams of material was collected (88% yield) with a melting point of138-140° C. The structure of the molecule was confirmed by NMR.

EXAMPLE 12 Synthesis of 1,4 Bis(4′-hydroxybenzoyloxy)t-butylphenylene

[0123] 1,4 Bis(4-methoxybenzoyloxy) t-butylphenylene (0.5 g., 0.00115mole) and aluminum chloride (1.23 g., 0.00921 mole) were added to ethanethiol (2.5 ml) and dichloromethane (2.5 ml) to form a slightly yellowsolution. This mixture was stirred for 1 hour and a white solidprecipitated out of solution during this time. The mixture wasprecipitated into 200 ml of water and extracted with ethyl ether. Theether was evaporated and 0.432 grams were recovered, (92% yield). Themelting point was not determined, but was found in be in excess of 280°C.

EXAMPLE 13 Synthesis of 1,4Bis(4″-(4′-methoxybenzoyloxy)benzoyloxy)t-butylphenylene

[0124] The dark orange solution of anisoyl chloride (0.357 g, 2.096mmole), 1,4 bis(4′-methoxybenzoyloxy) t-butylphenylene (0.355 g, 0.873mmole) in pyridine (25 ml) and triethyl amine (0.5 ml) were stirredunder nitrogen for 4 hr. The pyridine was removed under vacuum, and themixture was extracted into ethyl ether (200 ml). Amine hydrochloride andthe product were insoluble and were removed by vacuum filtration. Theamine hydrochloride was removed by washing the solids with water andacetone. The product had a melting point of 222-224° C. and thestructure of the molecule was confirmed by NMR.

EXAMPLE 14 Synthesis of 1,4Bis(4′-methacryloylbenzoyloxy)t-butyphenylene and 1-(hydroxybenzoyloxy),4-(4′-methacryloylbenzoyloxy)t-butylphenylene

[0125] 0.2 g (4.92×10⁻⁴ mole) 1,4 bis(4′-hydroxybenzoyloxy)t-butylphenylene was dissolved in 1 ml pyridine containing 10 ppmbenzophenone, and to this was slowly added 0.026 ml (2.46×10 mole)methacryloyl chloride dissolved in 2 ml methylene chloride. Afterstirring for 12 hours at room temperature, the methylene chloride waspumped off and the remaining pyridine solution was diluted into 0.1 NHCl to neutralize the pyridine and precipitate the product. Afterwashing the precipitate with water and drying under vacuum, theprecipitate was taken up into ether and dried with MgSO₄. After etherevaporation, the suspension was taken up into 3 ml methylene chloride inwhich the starting diphenol was insoluble. After filtering away thediphenol, the monomethacrylate (T_(m)=230° C.) was crystallized from theremaining solution at room temperature by the addition of 3 ml hexane.The remaining clear solution contained mainly the dimethacrylate in verysmall amounts (T_(m)=142° C.).

EXAMPLE 15 Synthesis ofbis-(4-{2-tert-butyl-4-[4-(2-methyl-acryloyloxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester{C0[H,TB,H](MeAcry)(O)}₂

[0126] In order to make decanedioic acidbis-(4-{2-tert-butyl-4-[4-(2-methyl-acryloyloxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester{CO[H,TB,H](MeAcry)(O)}₂(seb), 0.95 g, 1.95 mmole of 1-(hydroxybenzoyloxy),4-(4′-methacryloylbenzoyloxy) t-butylphenylene was dissolved in 10 mldry pyridine under dry nitrogen and then diluted with 20 ml drymethylene chloride. 0.233 g sebacoyl chloride (0.975 mmol) was dissolvedin 10 ml dry methylene chloride containing 10 ppm benzoquinone inhibitorand added slowly with syringe through a suba seal into the firstsolution with stirring. After 29 hours at room temperature a smallamount of precipitate was seen and the methylene chloride was pumped offand 0.01 g paradimethylaminopyridine was added as a catalyst to continuethe reaction.

[0127] After another 24 hours at room temperature, some unconvertedphenol was still observed by TLC and 0.5 ml methacryloyl chloride wasdissolved in 10 ml dry methylene chloride and added to the reactionmixture to react any unconverted starting material to thedimethacrylate. After 3 hours the phenol had been completely convertedand methylene chloride was removed under vacuum.

[0128] 100 ml of water containing 7.5 ml concentrated HCl was added tothe flask with stirring and stirred for four hours to remove thepyridine as the hydrochloride salt (pH=4). The water layer could bepoured from the white layer which stuck to the walls of the vessel.After washing once more with deionized water, 100 ml methylene chloridewas added to dissolve the solid and the resulting organic phase wastransferred to a separatory funnel and washed twice with 100 ml brinesaturated water and dried with magnesium sulfate. One gram each ofsilica and basic alumina were added to absorb any remaining methacrylicacid or carboxylic acid terminated products.

[0129] After standing for 8 hours the methylene chloride solution wasfiltered and added to 500 ml of stirred hexane. After 8 hours the pureprecipitated product was collected; the supernatent containedmethacrylated starting material.

[0130] The white precipitate eluted in 80/20 ether/hexane on silica as amajor spot and a very faint following spot. NMR revealed about 95%purity of the desired product (30% yield) with the rest being a methoxyterminated product which was carried over from the diphenol startingmaterial. Solutions could be cast into a translucent, nematic glass atroom temperature which gradually softened upon heating.

EXAMPLE 16 Synthesis of Decanedioic Acidbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester

[0131] 18.25 g, (44.9 mmole) of 1,4 bis(4′-hydroxybenzoyloxy)t-butylphenylene was dissolved in 120 ml dry pyridine under dry nitrogenand then diluted with 100 ml dry methylene chloride. 10.34 g sebacoylchloride (5.60 mmol) was dissolved in 20 ml dry methylene chloride andadded slowly with syringe through a suba seal into the first solutionwith stirring. After 24 hours at room temperature a small amount ofprecipitate was seen and the methylene chloride and pyridine were pumpedoff

[0132] 300 ml of water containing 7.5 ml concentrated HCl was added tothe flask with stirring and stirred for four hours to remove thepyridine as the hydrochloride salt (pH=4). The water was filtered offfrom the white precipitate that formed in the vessel. 200 ml of acetonewas added to dissolve the mixture which was then stirred with 3 grams ofmagnesium sulfate to remove any remaining water, after which thesolution was dried down. 200 ml methylene chloride (DCM) was added todissolve the solid. After 24 hours at room temperature the unreacted 1,4bis(4′-hydroxybenoyloxy)t-butylphenylene crystallized out of solution asa white precipitate. The solution was then placed in the freezerovernight and decanedioic acidbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester precipitated out of solution.

[0133] The white precipitate eluted in 90/10 DCM/acetone on silica as amajor spot and a very faint spots resulting from higher orderpolymerization. The product had a high NMR purity (>95%).

EXAMPLE 17 Synthesis of Decanedioic Acidbis-(4-{2-tert-butyl-4-[4-(2-methyl-acryloyloxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester

[0134] 0.85 g, (0.868 mmole) of decanedioic acidbis-(4-{2-tert-butyl-4-[4-(hydroxy)-benzoyloxy]-phenoxycarbonyl}-phenyl)ester was dissolved in 20 ml dry pyridine under dry nitrogen and thendiluted with 20 ml dry methylene chloride. 0.118 g methacrylol chloride(1.13 mmol) was dissolved in 10 ml dry methylene chloride containing 10ppm benzoquinone inhibitor and added slowly with syringe through a subaseal into the first solution with stirring. After 24 hours at roomtemperature a small amount of precipitate was seen and the methylenechloride and pyridine were pumped off.

[0135] 100 ml of water containing 1.0 ml concentrated HCl was added tothe flask with stirring and stirred for two hours to remove the pyridineas the hydrochloride salt (pH=4). The water layer could be poured fromthe white layer, which stuck to the walls of the vessel. After washingonce more with deionized water. 50 ml methylene chloride was added todissolve the solid and the resulting organic phase was transferred to aseparatory funnel and washed twice with 100 ml brine saturated water anddried with magnesium sulfate. One gram each of silica and basic aluminawere added to absorb any remaining methacrylic acid or carboxylic acidterminated products. NMR revealed that the product was the desireddialkene terminated monomer.

EXAMPLE 18 Bis 1,4[4-hydroxybenzoyloxy]2-phenyl-phenylene

[0136] (100 g, 0.537 mole) of phenylhydroquinone and (229 g, 1.342 mole)of anisoyl chloride were added to 100 ml of pyridine and 500 ml of drydichloromethane. The mixture was stirred for 72 hours at roomtemperature under nitrogen gas until it was mostly solidified. The 1,4bis [4-methoxybenzoyl 2-phenyl phenylene] was recrystallized fromisopropyl alcohol for a 96% yield.

[0137] (42.72 g, 0.094 mole) of the 1,4 bis [4-methoxybenzoyl]2-phenylphenylene was added to a solution consisting of (100 g, 0.749 mole) ofaluminum chloride, (58.21 g, 0.937 mole) of ethane thiol and (199.04 g,2.344 mole) of dichloromethane. After one hour the reaction was quenchedwith 250 ml of isopropyl alcohol. The solids were filtered and theproduct 1,4 bis [4-hydroxybenzoyl] 2-phenyl phenylene was purified byextraction of the solid material with water and dichloromethane for a68.6% yield. It is suspected that the isopropyl alcohol partiallysolubilizes the product and yield was lost in the filtration of theprecipitated material. NMR was used to confirm the structure and purityof the material.

EXAMPLE 19 Bis 1,4[4-hydroxybenzoyloxy]2-methyl phenylene

[0138] (29 g, 0.23 mole) of methylhydroquinone and (100 g, 0.58 mole) ofanisoyl chloride were added to 50 ml of pyridine and 250 ml of drydichloromethane. The mixture was stirred for 72 hours at roomtemperature under nitrogen gas until it was mostly solidified. The 1,4bis [4-methoxybenzoyl] 2-methyl phenylene was recrystallized fromisopropyl alcohol for a 95% yield. (m.p. 172-174° C.)

[0139] (90 g, 0.229 mole) of the 1,4 bis [4-methoxybenzoyl 2-methylphenylene] was added to a solution consisting of (250 g, 1.835 mole) ofaluminum chloride, (142.27 g, 2.290 mole) of ethane thiol and (486 g,5.725 mole) of dichloromethane. After one hour the reaction was quenchedwith 880 ml of isopropyl alcohol. The solids were filtered and theproduct 1,4 bis [4-hydroxybenzoyl}2-methyl phenylene]was purified byextraction of the solid material with water and dichloromethane for an84% yield. NMR was used to confirm the structure and purity of thematerial.

[0140] Persons of ordinary skill in the art will recognize that manymodifications may be made to the present invention without departingfrom the spirit and scope of the present invention. The embodimentdescribed herein is meant to be illustrative only and should not betaken as limiting the invention, which is defined in the followingclaims.

I claim:
 1. A method for producing platform molecules comprising:providing a first phenylene ring comprising a first functional group ata para-position to a second functional group; providing a secondphenylene ring comprising a third functional group at a para-position toa fourth functional group; providing a third phenylene ring comprising adesired substituent and comprising a first functionality at apara-position to a second functionality; and reacting said firstfunctional group with said first functionality, producing at least afirst ester bond between said first phenylene ring and said thirdphenylene ring; and reacting said third functional group with said thirdfunctionality, producing at least a second ester bond between saidsecond phenylene ring and said third phenylene ring, thereby producingplatform molecules comprising a first terminal functionality at positionpara- to said first intervening ester bond and a second terminalfunctionality at a position para- to said second intervening ester bond,wherein at least one functionality selected from the group consisting ofsaid first terminal functionality and said second terminal functionalityis other than a polymerizable group; wherein, when both said firstterminal functionality and said second functionality are polymerizablegroups, said desired substituent provides sufficient steric hindrance toachieve a nematic state at room temperature while suppressingcrystallinity at room temperature.
 2. The method of claim 1 wherein bothsaid first terminal functionality and said second terminal functionalityare other than polymerizable groups.
 3. A method for producing platformmolecules comprising: providing a first phenylene ring comprising afirst functional group at a para-position to a second functional group;providing a second phenylene ring comprising third functional group at apara-position to a fourth functional group; providing a third phenylenering comprising a desired substituent and comprising a first hydroxylgroup at a para-position to a second hydroxyl group; and reacting saidfirst hydroxyl group with said first functional group, producing atleast a first ester bond between said first phenylene ring and saidthird phenylene ring; and reacting said second hydroxyl group with saidthird functional group, producing at least a second ester bond betweensaid second phenylene ring and said third phenylene ring, therebyproducing platform molecules comprising a first terminal functionalityat position para- to said first ester bond and a second terminalfunctionality at a position para- to said second ester bond, wherein atleast one functionality selected from the group consisting of said firstterminal functionality and said second terminal functionality is otherthan a polymerizable group; wherein, if one of said first terminalfunctionality or said second terminal functionality is a polymerizablegroup; wherein, if both said first terminal functionality and saidsecond functionality are polymerizable groups, said desired substituentprovides sufficient steric hindrance to achieve a nematic state at roomtemperature while suppressing crystallinity at room temperature.
 4. Themethod of claim 3 wherein both said first terminal functionality andsaid second terminal functionality are other than polymerizable groups.5. The method of claim 1 wherein said desired substituent is selectedfrom the group consisting of a methyl group and a t-butyl group.
 6. Themethod of claim 2 wherein said desired substituent is selected from thegroup consisting of a methyl group and a t-butyl group.
 7. The method ofclaim 3 wherein said desired substituent is selected from the groupconsisting of a methyl group and a t-butyl group.
 8. The method of claim4 wherein said desired substituent is selected from the group consistingof a methyl group and a t-butyl group.
 9. The method of claim 1 whereinsaid desired substituent is selected from the group consisting of alkylgroups having from about 1 to 6 carbon atoms and aryl groups.
 10. Themethod of claim 1 wherein said desired substituent is selected from thegroup consisting of alkyl groups having from about 1 to about 4 carbonatoms and phenyl groups.
 11. The method of claim 1 wherein said desiredsubstituent is selected from the group consisting of methyl groups,t-butyl groups, isopropyl groups, secondary butyl groups, and phenylgroups.
 12. The method of claim 2 wherein said desired substituent isselected from the group consisting of alkyl groups having from about 1to 6 carbon atoms and aryl groups.
 13. The method of claim 2 whereinsaid desired substituent is selected from the group consisting of alkylgroups having from about 1 to about 4 carbon atoms and phenyl groups.14. The method of claim 2 wherein said desired substituent is selectedfrom the group consisting of methyl groups, t-butyl groups, isopropylgroups, secondary butyl groups, and phenyl groups.
 15. The method ofclaim 3 wherein said desired substituent is selected from the groupconsisting of alkyl groups having from about 1 to 6 carbon atoms andaryl groups.
 16. The method of claim 3 wherein said desired substituentis selected from the group consisting of alkyl groups having from about1 to about 4 carbon atoms and phenyl groups.
 17. The method of claim 3wherein said desired substituent is selected from the group consistingof methyl groups, t-butyl groups, isopropyl groups, secondary butylgroups, and phenyl groups.
 18. The method of claim 1 wherein said secondfunctional group and said fourth functional group are selected from thegroup consisting of carboxyl groups and reactive derivatives of carboxylgroups.
 19. The method of claim 2 wherein said second functional groupand said fourth functional group are selected from the group consistingof carboxyl groups and reactive derivatives of carboxyl groups.
 20. Themethod of claim 3 wherein said second functional group and said fourthfunctional group are selected from the group consisting of carboxylgroups and reactive derivatives of carboxyl groups.
 21. The method ofclaim 4 wherein said second functional group and said fourth functionalgroup are selected from the group consisting of carboxyl groups andreactive derivatives of carboxyl groups.
 22. The method of claim 7wherein said second functional group and said fourth functional groupare selected from the group consisting of carboxyl groups and reactivederivatives of carboxyl groups.
 23. The method of claim 16 wherein saidsecond functional group and said fourth functional group are selectedfrom the group consisting of carboxyl groups and reactive derivatives ofcarboxyl groups.
 24. The method of claim 17 wherein said secondfunctional group and said fourth functional group are selected from thegroup consisting of carboxyl groups and reactive derivatives of carboxylgroups.
 25. The method of claim 18 wherein said second functional groupand said fourth functional group are selected from the group consistingof carboxyl groups and reactive derivatives of carboxyl groups.
 26. Themethod of claim 1 further comprising forming a mixture comprising saidplatform molecules and at least a fourth phenylene ring comprising afifth functional group at a position para- to a sixth functional group;and exposing said mixture to conditions effective to form at least athird ester bond between said fourth phenylene ring and a ring selectedfrom the group consisting of said first phenylene ring and said secondphenylene ring, thereby producing elongated platform moleculescomprising at least four phenylene rings and comprising a new terminalfunctionality at a position para- to said third ester bond.
 27. A methodfor producing polymerizable mesogens comprising: forming a mixturecomprising first phenylene rings comprising a first functional group ata position para-to a second functional group; second phenylene ringscomprising a third functional group at a position para- to a fourthfunctional group; and third phenylene rings comprising a desiredsubstituent and comprising a first functionality at a position para- toa second functionality; and exposing said mixture to conditionseffective to react said first functional group and said firstfunctionality, forming a first ester bond between said first phenylenering and said third phenylene ring, and to react said second functionalgroup and said third functionality forming a second ester bond betweensaid second phenylene ring and said third phenylene ring, therebyproducing platform mesogens comprising a first terminal functionality ata position para- to said first ester bond and a second terminalfunctionality at a position para- to said second ester bond, wherein atleast one functionality selected from the group consisting of said firstterminal functionality and said second terminal functionality is otherthan a polymerizable group; and reacting at least one of said first andsecond terminal functionalities with a polymerizable group, producingpolymerizable mesogens; wherein, when both said first terminalfunctionality and said second functionality are polymerizable groups,said desired substituent provides sufficient steric hindrance to achievea nematic state at room temperature while suppressing crystallinity atroom temperature.
 28. A method for producing polymerizable mesogenscomprising: forming a mixture comprising first phenylene ringscomprising a first functional group at a position para- to a secondfunctional group; second phenylene rings comprising a third functionalgroup at a position para- to a fourth functional group; and thirdphenylene rings comprising a desired substituent and comprising a firsthydroxyl group at a position para- to a second hydroxyl group; andexposing said mixture to conditions effective to react said firsthydroxyl group and said first functional group, forming a first esterbond between said first phenylene ring and said third phenylene ring,and to react said second hydroxyl group and said third functional groupforming a second ester bond between said second phenylene ring and saidthird phenylene ring, producing platform mesogens comprising a firstterminal functionality at a position para- to said first interveningester bond and a second terminal functionality at a position para- tosaid second ester bond, wherein at least one functionality selected fromthe group consisting of said first terminal functionality and saidsecond terminal functionality is other than a polymerizable group; andreacting at least one of said first and second terminal functionalitieswith a polymerizable group, producing polymerizable mesogens; wherein,when both said first terminal functionality and said secondfunctionality are polymerizable groups, said desired substituentprovides sufficient steric hindrance to achieve a nematic state at roomtemperature while suppressing crystallinity at room temperature.
 29. Themethod of claim 27 wherein both said first terminal functionality andsaid second functionality are other than polymerizable groups.
 30. Themethod of claim 28 wherein both said first terminal functionality andsaid second functionality are other than polymerizable groups.
 31. Themethod of claim 27 wherein said desired substituent is selected from thegroup consisting of a methyl group and a t-butyl group.
 32. The methodof claim 28 wherein said desired substituent is selected from the groupconsisting of a methyl group and a t-butyl group.
 33. The method ofclaim 29 wherein said desired substituent is selected from the groupconsisting of a methyl group and a t-butyl group.
 34. The method ofclaim 30 wherein said desired substituent is selected from the groupconsisting of a methyl group and a t-butyl group.
 35. The method ofclaim 27 wherein said desired substituent is selected from the groupconsisting of alkyl groups having from about 1 to 6 carbon atoms andaryl groups.
 36. The method of claim 27 wherein said desired substituentis selected from the group consisting of alkyl groups having from about1 to about 4 carbon atoms and phenyl groups.
 37. The method of claim 27wherein said desired substituent is selected from the group consistingof methyl groups, t-butyl groups, isopropyl groups, secondary butylgroups, and phenyl groups.
 38. The method of claim 28 wherein saiddesired substituent is selected from the group consisting of alkylgroups having from about 1 to 6 carbon atoms and aryl groups.
 39. Themethod of claim 28 wherein said desired substituent is selected from thegroup consisting of alkyl groups having from about 1 to about 4 carbonatoms and phenyl groups.
 40. The method of claim 28 wherein said desiredsubstituent is selected from the group consisting of methyl groups,t-butyl groups, isopropyl groups, secondary butyl groups, and phenylgroups.
 41. The method of claim 29 wherein said desired substituent isselected from the group consisting of alkyl groups having from about 1to 6 carbon atoms and aryl groups.
 42. The method of claim 29 whereinsaid desired substituent is selected from the group consisting of alkylgroups having from about 1 to about 4 carbon atoms and phenyl groups.43. The method of claim 29 wherein said desired substituent is selectedfrom the group consisting of methyl groups, t-butyl groups, isopropylgroups, secondary butyl groups, and phenyl groups.
 44. The method ofclaim 30 wherein said desired substituent is selected from the groupconsisting of alkyl groups having from about 1 to 6 carbon atoms andaryl groups.
 45. The method of claim 30 wherein said desired substituentis selected from the group consisting of alkyl groups having from about1 to about 4 carbon atoms and phenyl groups.
 46. The method of claim 30wherein said desired substituent is selected from the group consistingof methyl groups, t-butyl groups, isopropyl groups, secondary butylgroups, and phenyl groups.
 47. The method of claim 27 wherein saidsecond functional group and said fourth functional group are selectedfrom the group consisting of carboxyl groups and reactive derivatives ofcarboxyl groups.
 48. The method of claim 28 wherein said secondfunctional group and said fourth functional group are selected from thegroup consisting of carboxyl groups and reactive derivatives of carboxylgroups.
 49. The method of claim 29 wherein said second reactive groupand said fourth functional group are selected from the group consistingof carboxyl groups and reactive derivatives of carboxyl groups.
 50. Themethod of claim 30 wherein said second reactive group and said fourthfunctional group are selected from the group consisting of carboxylgroups and reactive derivatives of carboxyl groups.
 51. The method ofclaim 31 wherein said second reactive group and said fourth functionalgroup are selected from the group consisting of carboxyl groups andreactive derivatives of carboxyl groups.
 52. The method of claim 32wherein said second reactive group and said fourth functional group areselected from the group consisting of carboxyl groups and reactivederivatives of carboxyl groups.
 53. The method of claim 35 wherein saidsecond reactive group and said fourth functional group are selected fromthe group consisting of carboxyl groups and reactive derivatives ofcarboxyl groups.
 54. The method of claim 40 wherein said second reactivegroup and said fourth functional group are selected from the groupconsisting of carboxyl groups and reactive derivatives of carboxylgroups.
 55. The method of claim 27 further comprising: forming a mixturecomprising said platform molecules and at least fourth phenylene ringscomprising a fifth functional group at a para-position to a sixthfunctional group; and exposing said mixture to conditions effective toform at least a third ester bond between said fourth phenylene ring anda ring selected from the group consisting of said first phenylene ringand said second phenylene ring, thereby producing elongated platformmolecules comprising at least four phenylene rings and comprising a newterminal functionality at a position para- to said third ester bond. 56.The method of claim 27 wherein said polymerizable group comprises apolymerizable unsaturated carbon-carbon bond.
 57. The method of claim 27further comprising reacting a first terminal functionality of a firstplatform molecule with a first end of a bridging agent selected from thegroup consisting of an α,ω-carboxylic acid and an oligodialkylsiloxanecomprising an alkyl group having from about 4 to about 12 carbon atoms;and reacting a first terminal functionality of a second platformmolecule with a second, opposed end of said bridging agent.
 58. A methodcomprising: providing hydroquinone comprising a desired substituent andcomprising a first hydroxyl group at a para-position to a secondhydroxyl group; exposing said hydroquinone to 1-(4-chloroalkyloxy)benzoyl chloride under conditions effective to form a first esterlinkage between said first hydroxyl group and a first benzoyl group of afirst 1-(4-chloroalkyloxy) benzoyl chloride molecule and to form asecond ester linkage between said second hydroxyl group and a secondbenzoyl group of a second 1-(4-chloroalkyloxy) benzoyl chloridemolecule, thereby forming a bis-chloro compound comprising bischlorogroups and a central phenylene ring bearing said desiredsubstituent; wherein, when said bis-chloro groups are converted topolymerizable groups, said desired substituent provides sufficientsteric hindrance to achieve a nematic state at room temperature whilesuppressing crystallinity at room temperature.
 59. The method of claim58 wherein said desired substituent is selected from the groupconsisting of a methyl group and a t-butyl group.
 60. The method ofclaim 58, wherein said 1-(4-chloroalkyloxy) benzoyl chloride is producedby: forming a mixture comprising α,ω-substituted alkane comprising anα-hydroxyl group at a first end and an ω-hydroxyl group at an opposedend, and 4-nitrobenzoic acid comprising a nitro-group and a carboxylgroup at a position para- to said nitro-group; exposing said mixture toconditions effective to form an ester linkage between said carboxylgroup and an entity selected from the group consisting of saidα-hydroxyl group and said ω-hydroxyl group, producing4-(1-hydroxyalkyloxy)benzoic acid(1-hydroxyalkylester) and dimersthereof; and hydrolyzing said dimers, producing 4-(1-hydroxyalkyloxybenzoic acid) comprising 1-hydroxy and a benzoic acid hydroxy moiety;exposing said 4-(1-hydroxyaklyloxy)benzoic acid to a source of chlorineatoms effective to replace said 1-hydroxy and said benzoic acid hydroxymoiety, thereby producing said 1-(4-chloroalkyloxy) benzoyl chloride.61. The method of claim 58 further comprising hydrolyzing said chlorineatoms from said bis chloro compound to produce platform moleculescomprising at least one hydroxyalkyloxy terminal functionality.
 62. Themethod of claim 59 further comprising hydrolyzing said chlorine atomsfrom said bis chloro compound to produce platform molecules comprisingat least one hydroxyalkyloxy terminal functionality.
 63. The method ofclaim 60 further comprising hydrolyzing said chlorine atoms from saidbis chloro compound to produce a platform molecules comprising at leastone hydroxyalkyloxy terminal functionality.
 64. The method of claim 61further comprising collecting said platform molecules.
 65. The method ofclaim 62 further comprising collecting said platform molecules.
 66. Themethod of claim 63 further comprising collecting said platformmolecules.
 67. The method of claim 61 further comprising reacting saidhydroxyalkyloxy terminal functionality with a polymerizable group. 68.The method of claim 62 further comprising reacting said hydroxyalkyloxyterminal functionality with a polymerizable group.
 69. The method ofclaim 63 further comprising reacting said hydroxyalkyloxy terminalfunctionality with a polymerizable group.
 70. The method of claim 64further comprising reacting said hydroxyalkyloxy terminal functionalitywith a polymerizable group.
 71. The method of claim 65 furthercomprising reacting said hydroxyalkyloxy terminal functionality with apolymerizable group.
 72. The method of claim 66 further comprisingreacting said hydroxyalkyloxy terminal functionality with apolymerizable group.
 73. The method of claim 61 further comprisingreacting a first hydroxyalkyloxy end group on a first platform moleculewith an α-end of a bridging agent comprising an α,ω-carboxylic acid; andreacting a second hydroxyalkyloxy end group on a second platformmolecule with a ω-end of said bridging agent.
 74. The method of claim 62further comprising reacting a first hydroxyalkyloxy end group on a firstplatform molecule with an α-end of a bridging agent comprising anα,ω-carboxylic acid; and reacting a second hydroxyalkyloxy end group ona second platform molecule with a ω-end of said bridging agent.
 75. Themethod of claim 63 further comprising reacting a first hydroxyalkyloxyend group on a first platform molecule with an α-end of a bridging agentcomprising an α,ω-carboxylic acid; and reacting a second hydroxyalkyloxyend group on a second platform molecule with a ω-end of said bridgingagent.
 76. The method of claim 64 further comprising reacting a firsthydroxyalkyloxy end group on a first platform molecule with an α-end ofa bridging agent comprising an α,ω-carboxylic acid; and reacting asecond hydroxyalkyloxy end group on a second platform molecule with aω-end of said bridging agent.
 77. The method of claim 1 furthercomprising reacting a moiety selected from the group consisting of afirst hydroxyl group and a first hydroxyalkyloxy end group on a firstplatform molecule with an α-end of a bridging agent comprising anα,ω-carboxylic acid; and reacting a moiety selected from the groupconsisting of a second hydroxyl moiety and a second hydroxyalkyloxy endgroup on a second platform molecule with a α-end of said bridging agent.78. The method of claim 2 further comprising reacting a moiety selectedfrom the group consisting of a first hydroxyl group and a firsthydroxyalkyloxy end group on a first platform molecule with an α-end ofa bridging agent comprising an α,ω-carboxylic acid; and reacting amoiety selected from the group consisting of a second hydroxyl moietyand a second hydroxyalkyloxy end group on a second platform moleculewith a ω-end of said bridging agent.
 79. The method of claim 3 furthercomprising reacting a moiety selected from the group consisting of afirst hydroxyl group and a first hydroxyalkyloxy end group on a firstplatform molecule with an α-end of a bridging agent comprising anα,ω-carboxylic acid; and reacting a moiety selected from the groupconsisting of a second hydroxyl moiety and a second hydroxyalkyloxy endgroup on a second platform molecule with a ω-end of said bridging agent.80. The method of claim 4 further comprising reacting a moiety selectedfrom the group consisting of a first hydroxyl group and a firsthydroxyalkyloxy end group on a first platform molecule with an α-end ofa bridging agent comprising an α,ω-carboxylic acid; and reacting amoiety selected from the group consisting of a second hydroxyl moietyand a second hydroxyalkyloxy end group on a second platform moleculewith a ω-end of said bridging agent.
 81. The method of claim 5 furthercomprising reacting a moiety selected from the group consisting of afirst hydroxyl group and a first hydroxyalkyloxy end group on a firstplatform molecule with an α-end of a bridging agent comprising anα,ω-carboxylic acid; and reacting a moiety selected from the groupconsisting of a second hydroxyl moiety and a second hydroxyalkyloxy endgroup on a second platform molecule with a ω-end of said bridging agent.82. The method of claim 8 further comprising reacting a moiety selectedfrom the group consisting of a first hydroxyl group and a firsthydroxyalkyloxy end group on a first platform molecule with an α-end ofa bridging agent comprising an α,ω-carboxylic acid; and reacting amoiety selected from the group consisting of a second hydroxyl moietyand a second hydroxyalkyloxy end group on a second platform moleculewith a ω-end of said bridging agent.
 83. The method of claim 8 furthercomprising reacting a moiety selected from the group consisting of afirst hydroxyl group and a first hydroxyalkyloxy end group on a firstplatform molecule with an α-end of a bridging agent comprising anα,ω-carboxylic acid; and reacting a moiety selected from the groupconsisting of a second hydroxyl moiety and a second hydroxyalkyloxy endgroup on a second platform molecule with a ω-end of said bridging agent.84. The method of claim 9 further comprising reacting a moiety selectedfrom the group consisting of a first hydroxyl group and a firsthydroxyalkyloxy end group on a first platform molecule with an α-end ofa bridging agent comprising an α,ω-carboxylic acid; and reacting amoiety selected from the group consisting of a second hydroxyl moietyand a second hydroxyalkyloxy end group on a second platform moleculewith a ω-end of said bridging agent.
 85. The method of claim 15 furthercomprising reacting a moiety selected from the group consisting of afirst hydroxyl group and a first hydroxyalkyloxy end group on a firstplatform molecule with an α-end of a bridging agent comprising anα,ω-carboxylic acid; and reacting a moiety selected from the groupconsisting of a second hydroxyl moiety and a second hydroxyalkyloxy endgroup on a second platform molecule with a ω-end of said bridging agent.86. The method of claim 17 further comprising reacting said firstterminal functionality on a first platform molecule with a first end ofa bridging agent comprising an oligodialkylsiloxane comprising an alkylgroup having from about 4 to about 12 carbon atoms; and, reacting saidfirst terminal functionality on a second platform molecule with asecond, opposed end of said bridging agent.
 87. A method for makingplatform molecules comprising reacting 4-alkoxy benzoyl chloride with ahydroquinone comprising a desired substituent (R²) under firstconditions effective to produce bis 1,4[4-alkoxybenzoyloxy]-R²-phenylenecomprising bis terminal alkoxy groups wherein, when both bis terminalalkoxy groups are converted to polymerizable groups, R² providessufficient steric hindrance to achieve a nematic state at roomtemperature while suppressing crystallinity at room temperature.
 88. Themethod of claim 87 wherein said 4-alkoxy benzoyl chloride is 4-methoxybenzoyl chloride.
 89. The method of claim 87 wherein said1,4[4-alkoxybenzoyloxy-R²-phenylene is subjected to second conditionseffective to cleave said bis terminal alkoxy groups, thereby producing asolution comprising diphenolic platform molecules comprising bisterminal hydroxyl groups.
 90. The method of claim 88 wherein said1,4[4-alkoxybenzoyloxy-R²-phenylene is subjected to second conditionseffective to cleave said bis terminal alkoxy groups, thereby producing asolution comprising diphenolic platform molecules comprising bisterminal hydroxyl groups.
 91. The method of claim 87 wherein said firstconditions comprise a solution comprising a hydrogen chloride scavengingagent.
 92. The method of claim 91 wherein said solution furthercomprises a trialkylamine.
 93. The method of claim 88 wherein said firstconditions comprise a solution comprising a hydrogen chloride scavengingagent.
 94. The method of claim 93 wherein said solution furthercomprises a trialkylamine.
 95. The method of claim 89 wherein said firstconditions comprise a solution comprising a hydrogen chloride scavengingagent.
 96. The method of claim 95 wherein said solution furthercomprises a trialkylamine.
 97. The method of claim 90 wherein said firstconditions comprise a solution comprising a hydrogen chloride scavengingagent.
 98. The method of claim 97 wherein said solution furthercomprises a trialkylamine.
 99. The method of claim 87 wherein R² isselected from the group consisting of methyl groups and t-butyl groups.100. The method of claim 87 wherein when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminal alkoxygroups is to be incorporated into a monomer, R² is selected from thegroup consisting of t-butyl groups, isopropyl groups, secondary butylgroups, methyl groups, and phenyl groups; and, when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene is to be incorporated into a dimer,R² is selected from the group consisting of bulky organic groups andgroups having a bulk less than methyl groups.
 101. The method of claim88 wherein R² is selected from the group consisting of methyl groups andt-butyl groups.
 102. The method of claim 88 wherein when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminal alkoxygroups is to be incorporated into a monomer, R² is selected from thegroup consisting of t-butyl groups, isopropyl groups, secondary butylgroups, methyl groups, and phenyl groups; and, when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene is to be incorporated into a dimer,R² is selected from the group consisting of bulky organic groups andgroups having a bulk less than methyl groups.
 103. The method of claim89 wherein R² is selected from the group consisting of a methyl groupand a t-butyl group.
 104. The method of claim 89 wherein when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminal alkoxygroups is to be incorporated into a monomer, R²is selected from thegroup consisting of t-butyl groups, isopropyl groups, secondary butylgroups, methyl groups, and phenyl groups; and, when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene is to be incorporated into a dimer,R² is selected from the group consisting of bulky organic groups andgroups having a bulk less than methyl groups.
 105. The method of claim90 wherein R² is selected from the group consisting of a methyl groupand a t-butyl group.
 106. The method of claim 90 wherein when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminal alkoxygroups is to be incorporated into a monomer, R² is selected from thegroup consisting of t-butyl groups, isopropyl groups, secondary butylgroups, methyl groups, and phenyl groups; and, when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene is to be incorporated into a dimer,R² is selected from the group consisting of bulky organic groups andgroups having a bulk less than methyl groups.
 107. The method of claim90 wherein said second conditions comprise exposing said bis terminalalkoxy groups to a mixture comprising a quantity of alkyl ether havingfrom about 1 to about 8 carbon atoms and an amount of an aliphatic thioleffective to dissolve a concentration of aluminum chloride in achlorinated solvent, said exposing taking place at a temperature and fora time effective to selectively cleave said bis terminal alkoxy groupsto produce complexes comprising said diphenolic platform moleculescomprising intact aromatic ester bonds and to cause said complexes toprecipitate out of said solution substantially as they are formed. 108.The method of claim 107 wherein said alkyl ethers have from about 1 toabout 4 carbon atoms.
 109. The method of claim 107 wherein said alkylether is methyl ether.
 110. The method of claim 107 wherein saidaliphatic thiol comprises an alkyl group having from about 1 to about 11carbon atoms.
 111. The method of claim 108 wherein said aliphatic thiolcomprises an alkyl group having from about 1 to about 11 carbon atoms.112. The method of claim 109 wherein said aliphatic thiol comprises analkyl group having from about 1 to about 11 carbon atoms.
 113. Themethod of claim 107 wherein said aliphatic thiol is ethane thiol. 114.The method of claim 108 wherein said aliphatic thiol is ethane thiol.115. The method of claim 109 wherein said aliphatic thiol is ethanethiol.
 116. The method of claim 107 wherein said quantity of alkyl etherand said amount of aliphatic thiol is effective to produce at least onemole of thiol per mole of alkyl ether.
 117. The method of claim 108wherein said quantity of alkyl ether and said amount of aliphatic thiolis effective to produce at least one mole of thiol per mole of alkylether.
 118. The method of claim 109 wherein said quantity of alkyl etherand said amount of aliphatic thiol is effective to produce at least onemole of thiol per mole of alkyl ether.
 119. The method of claim 113wherein said quantity of alkyl ether and said amount of aliphatic thiolis effective to produce at least one mole of thiol per mole of alkylether.
 120. The method of claim 116 wherein said quantity of alkyl etherand said amount of aliphatic thiol is effective to produce at least onemole of thiol per mole of alkyl ether.
 121. The method of claim 108wherein said quantity of alkyl ether and said amount of aliphatic thiolis effective to produce at least two moles of thiol per mole of alkylether.
 122. The method of claim 109 wherein said quantity of alkyl etherand said amount of aliphatic thiol is effective to produce at least twomoles of thiol per mole of alkyl ether.
 123. The method of claim 110wherein said quantity of alkyl ether and said amount of aliphatic thiolis effective to produce at least two moles of thiol per mole of alkylether.
 124. The method of claim 113 wherein said quantity of alkyl etherand said amount of aliphatic thiol is effective to produce at least twomoles of thiol per mole of alkyl ether.
 125. The method of claim 116wherein said quantity of alkyl ether and said amount of aliphatic thiolis effective to produce at least two moles of thiol per mole of alkylether.
 126. The method of claim 107 wherein said concentration ofaluminum chloride produces a ratio of said aluminum chloride to saidalkyl ether of 4:1 or more.
 127. The method of claim 108 wherein saidconcentration of aluminum chloride produces a ratio of said aluminumchloride to said alkyl ether of 4:1 or more.
 128. The method of claim109 wherein said concentration of aluminum chloride produces a ratio ofsaid aluminum chloride to said alkyl ether of 4:1 or more.
 129. Themethod of claim 110 wherein said concentration of aluminum chlorideproduces a ratio of said aluminum chloride to said alkyl ether of 4:1 ormore.
 130. The method of claim 113 wherein said concentration ofaluminum chloride produces a ratio of said aluminum chloride to saidalkyl ether of 4:1 or more.
 131. The method of claim 116 wherein saidconcentration of aluminum chloride produces a ratio of said aluminumchloride to said alkyl ether of 4:1 or more.
 132. The method of claim121 wherein said concentration of aluminum chloride produces a ratio ofsaid aluminum chloride to said alkyl ether of 4:1 or more.
 133. A methodfor making platform molecules comprising: reacting 4-alkoxy benzoylchloride with a hydroquinone comprising a desired substituent (R²) underfirst conditions comprising a solution comprising a hydrogen chloridescavenging agent effective to produce bis1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminal alkoxygroups; wherein, when both of said bis terminal alkoxy groups areconverted to polymerizable groups, R² provides sufficient sterichindrance to achieve a nematic state at room temperature whilesuppressing crystallinity at room temperature; subjecting said1,4[4-alkoxybenzoyloxy]-R²-phenylene to second conditions effective tocleave said bis terminal alkoxy groups, thereby producing a solutioncomprising diphenolic platform molecules comprising bis terminalhydroxyl groups, said second conditions comprising an amount ofchlorinated solvent comprising at least one mole of thiol per mole ofmethyl ether and comprising a concentration of aluminum chloride at amolar ratio of 4:1 or more to said methyl ether, said exposing occurringat a temperature and for a time effective to selectively cleave said bisterminal alkoxy groups to produce complexes comprising said diphenolicplatform molecules comprising intact aromatic ester bonds and to causesaid complexes to precipitate out of said solution substantially as theyare formed, said chlorinated solvent being present in an amounteffective to maintain said precipitate in slurry form.
 134. The methodof claim 133 further comprising quenching said precipitate.
 135. Themethod of claim 133 wherein said amount of chlorinated solvent comprisesa molar excess of from about 3 to about 7 relative to said ethane thiol.136. The method of claim 133 wherein said amount of chlorinated solventcomprises a molar excess of 5 or more relative to said ethane thiol.137. The method of claim 134 wherein said amount of chlorinated solventcomprises a molar excess of from about 3 to about 7 relative to saidethane thiol.
 138. The method of claim 134 wherein said amount ofchlorinated solvent comprises a molar excess of 5 or more relative tosaid ethane thiol.
 139. The method of claim 133 wherein said temperaturecomprises an initial temperature of about 0° C.
 140. The method of claim134 wherein said temperature comprises an initial temperature of about0° C.
 141. The method of claim 135 wherein said temperature comprises aninitial temperature of about 0° C.
 142. The method of claim 136 whereinsaid temperature comprises an initial temperature of about 0° C. 143.The method of claim 137 wherein said temperature comprises an initialtemperature of about 0° C.
 144. The method of claim 138 wherein saidtemperature comprises an initial temperature of about 0° C.
 145. Themethod of claim 133 wherein said chlorinated solvent is methylenechloride.
 146. The method of claim 134 wherein said chlorinated solventis methylene chloride.
 147. The method of claim 138 wherein saidchlorinated solvent is methylene chloride.
 148. The method of claim 144wherein said chlorinated solvent is methylene chloride.
 149. The methodof claim 133 wherein R² is selected from the group consisting of methylgroups and t-butyl groups.
 150. The method of claim 133 wherein whensaid bis 1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminalalkoxy groups is to be incorporated into a monomer, R² is selected fromthe group consisting of t-butyl groups, isopropyl groups, secondarybutyl groups, methyl groups, and phenyl groups; and, when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene is to be incorporated into a dimer,R² is selected from the group consisting of bulky organic groups andgroups having a bulk less than methyl groups.
 151. The method of claim134 wherein R² is selected from the group consisting of methyl groupsand t-butyl groups.
 152. The method of claim 134 wherein when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene comprising bis terminal alkoxygroups is to be incorporated into a monomer, R² is selected from thegroup consisting of t-butyl groups, isopropyl groups, secondary butylgroups, methyl groups, and phenyl groups; and, when said bis1,4[4-alkoxybenzoyloxy]-R²-phenylene is to be incorporated into a dimer,R¹ is selected from the group consisting of bulky organic groups andgroups having a bulk less than methyl groups.
 153. The method of claim 1wherein said first terminal functionality and said second terminalfunctionality are independently selected from the group consisting ofpolymerizable groups, hydroxyl groups, amino groups, sulfhydryl groups,halogen atoms, H—(CH₂)_(n)—O— groups, Cl(CH₂)_(n)—O— groups,Br(CH₂)_(n)—O— groups, I(CH₂)_(n)—O—, wherein n is from about 2 to about12 and CH₂ independently can be substituted by oxygen, sulfur, or anester group; provided that at least 2 carbon atoms separate said oxygenor said ester group.
 154. The method of claim 3 wherein said firstterminal functionality and said second terminal functionality areindependently selected from the group consisting of polymerizablegroups, hydroxyl groups, amino groups, sulfhydryl groups, halogen atoms,H—(CH₂)_(n)—O— groups, Cl(CH₂)_(n)—O— groups, Br(CH₂)_(n)—O— groups,I(CH₂)_(n)—O—, wherein n is from about 2 to about 12 and CH₂independently can be substituted by oxygen, sulfur, or an ester group;provided that at least 2 carbon atoms separate said oxygen or said estergroup.
 155. The method of claim 27 wherein said first terminalfunctionality and said second terminal functionality are independentlyselected from the group consisting of polymerizable groups, hydroxylgroups, amino groups, sulfhydryl groups, halogen atoms, H—(CH₂)_(n)—O—groups, Cl(CH₂)_(n)—O— groups, Br(CH₂)_(n)—O— groups, I(CH₂)_(n)—O—,wherein n is from about 2 to about 12 and CH₂ independently can besubstituted by oxygen, sulfur, or an ester group; provided that at least2 carbon atoms separate said oxygen or said ester group.
 156. The methodof claim 28 wherein said first terminal functionality and said secondterminal functionality are independently selected from the groupconsisting of polymerizable groups, hydroxyl groups, amino groups,sulfhydryl groups, halogen atoms, H—(CH₂)_(n)—O— groups, Cl(CH₂)_(n)—O—groups, Br(CH₂)_(n)—O— groups, I(CH2)_(n)—O—, wherein n is from about 2to about 12 and CH₂ independently can be substituted by oxygen, sulfur,or an ester group; provided that at least 2 carbon atoms separate saidoxygen or said ester group.